1. Field of the Invention
The present invention relates to a mounting structure with a heat sink for electronic component and securing members for the mounting structure and more particularly to the mounting structure with the heat sink for at least one electronic component suitably used for electronic components such as an LSI (Large Scale Integration) circuit whose temperature rises at a time of operations and the securing member suitably used for the mounting structure.
2. Description of the Related Art
Conventionally, a heat sink of this kind is used for suppressing a temperature rise caused by power consumed when an LSI in a package is operating by being in thermal contact with the LSI surface-mounted or insertion-mounted on a printed circuit board. When the heat sink is secured to the printed circuit board, since the LSI package is sandwiched between the heat sink and the printed circuit board, it is necessary that some contrivance to accommodate variations in a height of the LSI package is provided.
Variations in the height readily occur among the mounted LSI packages and reasons for the variations in the height of the LSI package are various. For example, in the case of a surface-mounting type LSI package such as a BGA (Ball Grid Array) or a like, variations in thickness of solder balls arranged in a grid-like form readily occurs among lots and, in the case of face-down bonding, variations occur easily, or also in an insertion-mounted type package, variations occurs readily in a degree to which a lead is inserted into a hole of a land portion.
A related mounting structure of this type is disclosed in, for example, Patent Reference 1 (Japanese Patent Application Laid-open No. 2001-057405) in which a heat transmission rubber is disposed between a heat sink and an LSI package so that variations in a direction of height are accommodated by the heat transmission rubber.
Also, a related mounting structure of this kind is disclosed in, for example, Patent Reference 2 (Japanese Patent Application Laid-open No. Hei 09-139450) in which each compression coil spring is attached to each male screw serving as a securing member and is made to accommodate variations in height in a direction of an LSI package. However, thermal conductivity of the heat transmission rubber is low and highly-priced, which causes high costs of the heat sink securing structure, therefore, conventionally, the compression springs are generally used.
FIG. 5 is an exploded perspective view showing a related mounting structure (structure for securing a heat sink) of an LSI package with a heat sink using compression coil springs and disassembled securing members to be used in the mounting structure. FIG. 6 is a side view showing the related mounting structure and the securing members for the mounting structure. FIG. 7 is the mounting structure of FIG. 5 taken along the line A-A.
The related mounting structure (structure for securing heat sink) of this type roughly includes, as shown in FIGS. 5, 6, and 7, a printed circuit board 1, an LSI package 2 being surface-mounted on the printed circuit board 1, a heat sink 3 having a fin structure disposed in a thermal contact state on an upper surface of the LSI package 2, a pair of lower and upper stiffeners (lower stiffener 4 and upper stiffener 5) to support and reinforce the printed circuit board 1, and a set of securing members including screw members 6, male screws 7, shafts 8, washers and a like, and compression coil springs 10 to accommodate variations in height in a direction of the LSI package 2 (refer to Patent Reference 3 [Japanese Patent Application Laid-open No. 2000-058703] for the stiffener structure) The pair of upper and lower stiffeners 4 and 5 is made of a metal plate and is secured with the screw members 6 in a state in which the printed circuit board 1 is sandwiched between the upper and lower stiffeners. The upper and lower stiffeners are secured to each other and, therefore, an occurrence of warpage caused by heat of the printed circuit board 1 can be prevented.
The above heat sink is so configured that many dissipating fins 3b to increase contact areas (dissipation area) with an outer atmosphere are disposed on an upper surface of an heat sink base 3a in parallel to one another and in an erected manner. At tour corners of the heat sink base 3a are formed through-holes 3c to insert the male screws 7. On the upper surface stiffeners 6, as shown in FIG. 7, is formed an aperture portion to allow the LSI package to be inserted. In positions corresponding to the through-holes 3c at four corners of the heat sink base 3a are attached, in a securing manner, the shafts 8 (female screws with upper portion being opened) to screw the male screws 7 therein. The LSI package 2 is so configured as to be exposed from the aperture portion 11 of the upper stiffener 5 and a flat upper surface of the LSI package 2 is in thermal contact with a bottom of the heat sink base 3a. 
Each of the above male screws 7 is passed through each of washers 9, each of compression coil springs 10 and each of through holes 3c in this order and, in this state, an end of each of the male screws is screwed into each of the upper stiffener 5 and, as a result, each of the male screws 7 is screw-secured to each of the shafts. Thus, conventionally, the heat sink 3 is screw-secured to the upper stiffener 5 using the screw members and the printed circuit board 1, in a state in which the LSI package is exposed from the aperture portion 11 of the upper stiffener 5 is sandwiched between the upper stiffener 5 and the lower stiffener 4 and is secured and, therefore, in a state in which the upper surface of the LSI package 2 is in thermal contact with the bottom of the heat sink base 3a, the heat sink 3 is secured to the printed circuit board 1.
According to the above configurations, even when variations in height occurs among the LSI packages 2 due to the easy occurrence of variations in the thickness of solder balls 12 (FIG. 7) arranged in a grid form and/or the easy occurrence of variations at a time of face-down bonding, each of the compression coil springs 10 sandwiched between a head portion of each of the male screws 7 and the upper surface of the heat sink base 3a, thereby variations in height can be accommodated (FIGS. 6 and 7).
However, the above related mounting structure has a problem. That is, the height accommodating tool to secure the heat sink 3 to the printed circuit board 1 is made up of the male screws 7, the shafts (female screws) 5, the compression coil springs 10, and the washers 9 and, therefore, component counts are large, many attaching man-hours are required, thus causing complicated mounting processes. More specifically, the related method for the heat sink requires, as shown in FIG. 5, (1) a process of attaching and securing each of the shafts (female screws) 8 to the upper stiffener 5, (2) a process of securing the upper stiffeners 5 and the lower stiffener 4 by using each of the screw members 6 with the printed circuit board 1 being sandwiched between the upper and lower stiffeners, (3) a process of attaching each of the washers 9 and each of the compression coil springs 10 to each of the male screws 7, and (4) a process of securing the heat sink 3 to the printed circuit board 1 by using each of the male screws 7 with the heat sink 3 being in thermal contact with the LSI package 2, thus resulting in complicated mounting work. Additionally, the related mounting structure for the heat sink has another problem. That is, each of the male screws 7 and/or compression coil springs 10 spring out therefrom, a wind path F (FIG. 6) of a fan (not shown) is stopped up, thus resulting in lowering of the dissipation efficiency.